It seems to have something to do with a verse by Layman Pang. From dialogue 2 in "The Sayings of Layman P'ang" translated by James Green:

What I do every day
Is nothing special:
I simply stumble around.
What I do is not thought out,
Where I go is unplanned.
No matter who tries to leave their mark,
The hills and dales are not impressed.Collecting firewood and carrying water
Are prayers that reach the gods.

In Ferguson's "Zen's Chinese Heritage" one finds in the section on Layman Pang, apparently from the same dialogue:

Pangyun then recited the verse whose last two lines are widely quoted:

How miraculous and wondrous,
Hauling water and carrying firewood.

There is not only nothingness because there is always, and always can manifest. - Thinley Norbu Rinpoche

ONE DAY SHIH-T'OU said to the Layman: "Since seeing me, what have your daily activities
been?"

"When you ask me about my daily activities, I can't open my mouth," the Layman replied.

"Just because I know you are thus I now ask you," said Shih-t'ou. Whereupon the Layman offered this verse:

My daily activities are not unusual,
I'm just naturally in harmony with them.
Grasping nothing, discarding nothing,
In every place there's no hindrance, no conflict.
Who assigns the ranks of vermilion and purple?
The hills' and mountains' last speck of dust
is extinguished.
[My] supernatural power and marvelous activity—
Drawing water and carrying firewood.

Shih-t'ou gave his assent. Then he asked: "Will you put on black robes or will you continue wearing
white?"

"I want to do what I like," replied the Layman. So he did not shave his head or dye his clothing.

It is relatively easy to accomplish the important matter of insight into one’s true nature, but uncommonly difficult to function freely and clearly [according to this understanding], in motion and in rest, in good and in adverse circumstances. Please make strenuous and vigorous efforts towards this end, otherwise all the teachings of Buddhas and patriarchs become mere empty words. - Torei